1. Field of the Invention
The present invention relates to a semiconductor device and a method of making the same, and more particularly to a semiconductor device which includes at least one well region having a substantially uniform surface impurity concentration, and a method of making the same.
2. Description of the Related Art
Referring to FIGS. 9 and 10, a well region of a conventional semiconductor device will be described.
As shown in FIG. 9, a semiconductor device such as conventional MOS type field effect transistors (MOSFETs) includes, for example, N-type semiconductor substrate 30, and P-type well region 31 provided in the substrate.
The P-type well region 31 is formed by ion-implantation of P-type impurity into the N-type semi-conductor substrate 30, followed by a thermal diffusion process. However, the well region thus formed exceeds a point b, which is a boundary of the impurity region introduced by the ion implantation, and reaches a point c, as shown in FIG. 9. As a result, the impurity concentration of the substrate surface (hereinafter called surface concentration) may be reduced at the boundary portion between the P-type well region 31 and the N-type semiconductor substrate 30 due to the impurity diffusion.
FIG. 10 is an impurity concentration profile in a surface direction of the semiconductor device. As illustrated in the drawing, the surface concentration of the P-type well region 31 is substantially uniform in a region defined between the center of the well region and the point a, but decreases in a region beyond the point a. Consequently, if a MOSFET is formed in the region between points a and c, the threshold voltage may be changed--for example, may be reduced to an undesired value. For this purpose, forming the MOSFET in the region between points a and c where the surface concentration is reduced is avoided, and the MOSFET is provided in a region of the p-type well region 31 inside the point a, thereby preventing the change in the threshold voltage.
In other words, the change in the threshold voltage is prevented by locating the MOSFET in the region having uniform surface concentration.
However, if it is intended to increase the impurity concentration of the P-type well region, or it is intended to increase the depth of the well region, the distance between point a and c where the surface concentration is reduced will be increased. Consequently, the region in which the MOSFET cannot be formed will be enlarged. This is a serious problem for semiconductor integrated circuits for requiring high integration density.